1. Mid term grade distribution

2. Recap from before spring break
Direct link networks
Packet switching

3. Internetworking

4. Internet
collection of networks, each of which can be different types of networks

5. IP Internet Protocol Stack view
The packet goes through different network protocols as it traverses different types of networks
R’s are routers and H is the hosts
Remember, bridges do not perform protocol translation and so wouldn’t show up in this figure. E.g., there may be a bridge between R1 and R2 R1 R2 R3 H1 H8
ETH
FDDI IP
TCP
PPP

6. Service Model Connectionless (datagram-based)
Each datagram carries the destination address
Best-effort delivery (unreliable service)
packets may be lost
packets can be delivered out of order
duplicate copies of a packet can be delivered
packets can be delayed for a long time

7. Issues
Global naming (IP addresses) and mechanisms to translate to human usable form (e.g. (DNS)
Fragmentation - not all networks can deal with a given packet size
Routing, mapping ip address to physical address etc..

8. Issue 1: Global Addresses
As we connect different types of networks, need a way to address each host, independent of the network type (ethernet, FDDI etc.)
Darwin.cc.nd.edu’s address is 08:00:20:7e:16:cc

Properties (
globally unique
hierarchical: network + host, so that we can use it to route

9. Issue 1: ipv4 address classes
Dot Notation to describe the 32 bit address:
Class D ( ) Multicast
Class E (240 – 247) Experimental

10. Darwin.cc.nd.edu Darwin.cc.nd.edu = 129.74.250.114
Class B address, ND owns X.X

11. Domain Name Service (DNS)
Provides Internet domain name to IP address translation
Domain name translation (nd.edu)
Hostname translation (wizard.cse.nd.edu)
Service location (MX records, mail service for ND)
Use nslookup command to interact with DNS

12. Domain Naming System Hierarchy
DNS is hierarchical

13. DNS hierarchy DNS name space is hierarchical:
- fully qualified names are “little endian”
- scalability
- decentralized administration
- domains are naming contexts
com
gov
org
generic TLDs
net
top-level
domains
(TLDs)
firm
shop
arts
web us fr
country-code TLDs
.edu nd
berkeley
duke
cse
chem cs
eecs cc
www
sys
Source: Jeff Chase

14. DNS Name Server Hierarchy
com
gov
org
net
firm
shop
arts
web us fr
Root servers list
servers for every TLD.
Subdomains correspond to organizational (admininstrative) boundaries, which are not necessarily geographical.
DNS servers are organized into a hierarchy that mirrors the name space.
Specific servers are designated as authoritative for portions of the name space.
.edu
Servers may delegate management of subdomains to child name servers.
duke
...
Servers are bootstrapped with pointers to selected peer and parent servers. nd cc
Parents refer subdomain queries to their children.
cse
chem
Resolvers are bootstrapped with pointers to one or more local servers; they issue recursive queries.
Source: Jeff Chase

15. Multicast DNS (Apple Rendezvous)
Situations where there are no DNS servers (local intranet - for example home users who want to name the machines without dealing with the complexities of maintaining DNS servers) or where you may not know where the DNS servers are
Create names in the .local domain. For example, my laptop can be called surendar.local.
Zeroconf initiative

16. IP v4 Datagram format

17. IP v6 format
Developed so that we can address more than 2^32 hosts (ipv4)
Version (4 bits) – only field to keep same position and name
Class (8 bits) – was Type of Service (TOS), renamed
Flow Label (20 bits) – new field
Payload Length (16 bits) – length of data, slightly different from total length
Next Header (8 bits) – type of the next header, new idea
Hop Limit (8 bits) – was time-to-live, renamed
Source address (128 bits)
Destination address (128 bits)

18. IPv4 and IPv6 headers
Remove GPN from headers

19. Basic Headers - IPV6 Simplifications
Fixed length of all fields, not like old options field – IHL, or header length irrelevant
Remove Header Checksum – rely on checksums at other layers
No hop-by-hop fragmentation – fragment offset irrelevant
MTU discovery (IPv4 also support Path MTU discovery)
Add extension headers – next header type (sort of a protocol type, or replacement for options)
Basic Principle: Routers along the way should do minimal processing
Discuss PMTU discovery because it is significantly different than IPv4

20. Issue 2: Fragmentation and Reassembly
Cannot expect all networks to deal with the same packet size, can choose the absolute smallest - but that would mean poor performance for all networks
Each network has some MTU (maximum transmission unit)
Design decisions
fragment when necessary (MTU < Datagram)
re-fragmentation is possible
fragments are self-contained datagrams
use CS-PDU (not cells) for ATM
delay reassembly until destination host
do not recover from lost fragments
try to avoid fragmentation at source host

21. Example Ident = x Start of header Rest of header 1400 data bytes
Offset = 0
(b)
512 data bytes 1
Offset = 64
376 data bytes
Offset = 128

22. Gateway13 example Ifconfig eth0 in gateway13.cse.nd.edu
Link encap:Ethernet HWaddr 00:07:E9:3C:8F:80
inet addr: Bcast: Mask:
inet6 addr: fe80::207:e9ff:fe3c:8f80/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1

23. Issue 3: Datagram Forwarding
Strategy
every datagram contains destination’s address
if connected to destination network, then forward to host
if not directly connected, then forward to some router
forwarding table maps network number into next hop
each host has a default router
each router maintains a forwarding table
Example (R2)
Network number
Next 1 R3 2 R1 3
Interface 1 4
Interface 0

24. Address Translation Map IP addresses into physical addresses
destination host
next hop router
Techniques
encode physical address in host part of IP address
table-based
Mechanism to map IP to physical address: ARP
table of IP to physical address bindings
broadcast request if IP address not in table
target machine responds with its physical address
table entries are discarded if not refreshed

25. ARP Details Request Format Notes
HardwareType: type of physical network (e.g., Ethernet)
ProtocolType: type of higher layer protocol (e.g., IP)
HLEN & PLEN: length of physical and protocol addresses
Operation: request or response
Source/Target-Physical/Protocol addresses
Notes
table entries timeout in about 10 minutes
update table with source when you are the target
update table if already have an entry
do not refresh table entries upon reference

26. ARP Packet Format 8 16 31 Hardware type = 1 ProtocolType = 0x08008 16 31
Hardware type = 1
ProtocolType = 0x0800
HLen = 48
PLen = 32
Operation
SourceHardwareAddr (bytes 0 ― 3)
SourceHardwareAddr (bytes 4 ― 5)
SourceProtocolAddr (bytes 0 ― 1)
SourceProtocolAddr (bytes 2 ― 3)
TargetHardwareAddr (bytes 0 ― 1)
TargetHardwareAddr (bytes 2 ― 5)
TargetProtocolAddr (bytes 0 ― 3)

27. Sample arp table in darwin.cc.nd.edu
arp -a
Net to Media Table: IPv4
Device IP Address Mask Flags Phys Addr
hme0 eafs-e06.gw.nd.edu :d0:c0:d3:aa:40
hme0 bind.nd.edu :00:20:8a:5f:cf
hme0 honcho-jr.cc.nd.edu :b0:d0:82:83:7f
hme0 mail-vip.cc.nd.edu :e0:52:0c:56:c4
hme0 john.helios.nd.edu :00:20:85:db:c4
hme0 casper.helios.nd.edu :00:20:b1:f8:e1
hme0 pinky.helios.nd.edu :00:20:a9:88:30

28. ARP problems ARP trusts any response - no authentication method
Works great at home, how about Notre Dame
Replies which do not correspond to requests are allowed to update cache in many instances
New information must supercede old info

29. Internet Control Message Protocol (ICMP)
Mechanisms to notify of errors (not mandatory)
Echo (ping)
Redirect (from router to source host)
Destination unreachable (protocol, port, or host)
TTL exceeded (so datagrams don’t cycle forever)
Checksum failed
Reassembly failed
Cannot fragment